Most rivers and streams experience natural variations in water flow throughout the year. Whilst climate change increases the risk of extreme floods and extreme droughts, ‘normal’ floods and droughts are part of the natural rhythm of river ecosystems.

Natural variations in water flow are important for many reasons. One is that they can ‘clean’ the river bottom. Although this can be devastating for the organisms which are ripped off by a flood, or killed by a drought, such ‘cleaning’ creates open spaces for colonisation by other organisms. Disturbance by floods and droughts is – in the long run – important for maintaining biodiversity in rivers.

However, rivers across the world are increasingly regulated to serve human needs, such as flood protection, transport, irrigation, hydropower generation and drinking water supply. In Norway, more than two-thirds of river basins are affected by regulation. This has altered natural flow dynamics. In winter, because more electricity is needed for heating, hydropower companies take more water from the reservoirs and release it into streams, generating electricity. As a result, many regulated rivers have relatively high discharges in winter. This is in contrast to unregulated rivers in Norway which normally have low discharges in winter because precipitation falls as snow and does not run off immediately.

Benthic algae are the basis of many food webs in rivers. Macroinvertebrates feed on algae and detritus, and are themselves an important food source for fish. In their study, the researchers asked: what are the consequences of an altered flow regime for macroinvertebrates and benthic algae?

Macroinvertebrate and benthic algal populations are commonly used as indicators to assess the health (or ecological status) of rivers. The researchers investigated whether these indicators could also be used in regulated rivers, and how river flow interacts with other ecosystem stressors like acidification or nutrient enrichment.

“Studying the effects of river flow on stream biota is not straightforward,” says Susanne Schneider, senior research scientist at NIVA, “Each stream is special in its own way, has a slightly different flow regime, sediment, water chemistry, shading, and so on, than its neighbour. The interaction of so many factors – which all affect macroinvertebrates and benthic algae – makes it difficult to detect the effects of flow among all other factors which also are important. Properly replicated and controlled experimental designs in streams are rarely possible in practice.”

The flume experiments in Norway. Image: NIVA

Varying water flows and nutrient levels in flume experiments

The researchers used artificial steel flumes to standardise conditions in their experiments as far as possible. “Flumes have the advantage that we can study the influence of a certain factor, – such as flow increase – while all other conditions are kept constant,” Schneider explains.

Researchers manipulated water flow and nutrient (nitrogen and phosphorus) supply in the flumes over time. They found that an increased nutrient supply led to an increased biomass of benthic algae within a few days. Algae ‘feed’ on nutrients, and more food leads to better growth.

“We also found that a moderate increase in flow caused an increase in benthic algal biomass,” says Schneider. “This may seem surprising, because we all know that large floods can ‘clean’ the river bottom, ripping benthic algae from the bed and washing them away. But a moderate increase in flow can have a positive effect on algal growth. This happens when increased flows aren’t strong enough to rip off the algae, but do transport more nutrients to the algal patches, leading to increased algal growth.”

Interestingly, when nutrients and water flow both were increased at the same time in the flumes, the effect on algal biomass was smaller than the sum of both individual effects. This was because a larger patch of benthic algae – caused by more nutrients – can more easily be ripped off already by a moderate increase in flow. “What we learn from this is that a flood in a nutrient-rich river will have different consequences than a flood in a nutrient-poor river,” Schneider explains.

Comparing flumes and fieldwork

The researchers found that their results varied between different flumes and the nearby river from where they took the water for their experiments. “This was because ‘controlled’ flumes can never really be the same as ‘real’ ecosystems,” Schneider says.
In order to compare their flume experiments with real-world findings, the scientists studied 32 regulated and 32 unregulated river sites in Norway and Germany. In their river experiments, none of the monitoring indices of macroinvertebrates and benthic algae used for ecosystem status assessment were affected by the water flow regime. This suggests that these indices can be applied to regulated rivers as well as non-regulated ones.

“We did, however, see some effect of the long-term flow regime, calculated from five years of discharge data, on the species composition of macroinvertebrates,” Schneider says. “We found that a flow regime which is comparatively stable over several years – for example in a regulated river – changes the species composition of macroinvertebrates towards those which prefer slowly flowing water. We also saw that less variable flow conditions lead to a reduced proportion of grazers – those species that directly feed on benthic algae – among the macroinvertebrates.”

Thus, a more uniform flow regime may lead to a higher biomass of benthic algae, via a direct and an indirect effect. Direct, because the occurrence of lower and fewer floods will ‘clean’ less algae from the river bottom; and indirect because fewer grazers will eat less algae.

Sampling on the Atna, an unregulated river in Norway. Image: NIVA

Similar ecological impacts of natural and artificial flow regimes?

Another finding that caught the scientists’ interest was that natural differences in flow regime had similar effects on the biota as those observed in artificially modified flow regimes. Macroinvertebrates and benthic algae responded to changed flow regimes, regardless of whether alteration was due to natural reasons, or caused by human regulation.

That climatic and natural variation in river flow affected benthic algae was something the researchers have seen before. Data collected over more than 20 years at the lake outlet Atna, and the headwater stream Li, both located in the Norwegian mountain area of Rondane, showed similar results. In this remote location, there is practically no human interference except climate change.

Swift recolonisation after extreme flow events

“In summary, we found that there are short-term effects of extreme events like floods and droughts on benthic algae and macroinvertebrates,” says Schneider. “Benthic algae generally were more affected by floods, while macroinvertebrates were more affected by droughts. Within a few weeks or months after extreme events, however, benthic algae and macroinvertebrates usually recolonise the rivers, and few long-term effects were apparent.”

“One prominent long-term effect was however that a new flow regime affected species composition of macroinvertebrates. Variability affected grazing macroinvertebrates – which in turn may lead to a higher biomass of benthic algae in rivers with stable flows,” Schneider added.

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